1. Field Of The Invention
This invention relates generally to a heterojunction bipolar transistor and, more particularly, to an InAlAs/InGaAlAs heterojunction bipolar transistor that includes a constant InGaAlAs quaternary collector layer, a graded-composition InGaAlAs emitter-base transition region, a graded-doping InGaAs base layer and/or a graded-composition InGaAlAs base layer.
2. Discussion of the Related Art
Because of certain known device performance advantages, heterojunction bipolar transistors (HBTs) are desirable for many applications, especially high power and high frequency microwave applications. Early HBTs were generally gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) HBTs. As device performance demands increased, GaAs/AlGaAs HBTs were limited in their ability to provide high enough frequency performance, acceptable cut-off frequency (f.sub.T) and high enough gain for amplifying purposes. In order to overcome these limitations, indium gallium arsenide/indium aluminum arsenide (InGaAs/InAlAs) HBTs that were matched to indium phoside (InP) substrates were devised. The advantages of the InAlAs/InGaAs HBTs over the GaAs/AlGaAs HBTs included higher electron saturation velocity and mobility, lower turn-on voltages, and larger bandgap discontinuity that provided high frequency performance advantages. However, original InAlAs/InGaAs HBTs suffered from a low collector breakdown voltage (BV.sub.CEO) and high output conductance that limited their use in high frequency microwave applications. In a typical example, an InGaAs collector had a bandgap of 0.75eV as compared with a GaAs collector having a bandgap of 1.45eV in an AlGaAs/GaAs HBT. Further, InGaAs/InAlAs HBTs experience a decrease in collector breakdown voltage BV.sub.CEO with increasing temperature.
A number of different concepts have been put forward in order to provide an acceptable collector breakdown voltage BV.sub.CEO and output conductance for InAlAs/InGaAs HBTs. For example, InAlAs/InGaAs HBTs have been developed that have collector layers of InP. See for example Stanchina, W.E., "Status and Potential of AlInAs/GaInAs/InP HBT ICs," Proc. Fourth Int. Conf. Indium Phosphide and Related Materials, pp. 434-447 (1992) and Nottenburg, R.N. et al, "High-speed InGaAs(P)/InP double-heterostructure bipolar transistors," IEEE Electron Device Lett., vol. EDL-8, pp. 282-284 (1983). Additionally, InAlAs/InGaAs HBTs having InAlAs collector layers have also been provided. See for example, Farley, C. W. et al. "Performance tradeoffs in AlInAs/GaInAs single-and double-heterojunction NPN heterojunction bipolar transistors," J. Vac. Sci. Technol., vol. B10, pp. 1023-1025 (1992). One reference has reported use of a graded InGaAlAs collector layer in an InAlAs/InGaAs HBT. See Vlcek, J. C. et all. "Multiply--graded InGaAs heterojunction bipolar transistors," Electron Lett., vol. 27, pp. 1213-1215 (1991). Further, composite InGaAs/InP collectors in HBTs have also been reported. The use of InP or composite InGaAs/InP collectors has been limited, however, because the growth of phosphorous based compounds is not compatible with most solidsource molecular beam epitaxial (MBE) systems. U.S. Pat. No. 5,150,185 issued Sep. 22, 1992 to Yamada discloses an HBT semiconductor device that appears to include a quaternary (InGaAs).sub.0.4 (InAlAs).sub.0.6 collector layer positioned between two n-type (InGaAs).sub.1-x (InAlAs).sub.x graded layers.
Although recent semiconductor HBT devices have had some success in providing low collector breakdown voltages and high output conductances, while at the same time providing high frequency performance, acceptable cut-off frequencies and high gain, other device criteria are also important for realizing increased device performance. For example, it is important that HBT devices have high base electron transit times so as to increase the switching speed and frequency of the device. U.S. patent application Ser. No. 07/876,199, filed Apr. 30, 1992, titled "Heterojunction Bipolar Transistor with Graded Base Doping" and assigned to the assignee of the instant application, discloses a heterojunction bipolar transistor that has graded base doping so as to increase the base transit time of the device. However, this is a GaAs/AlGaAs HBT device, and thus suffers from its ability to provide high frequency performance, etc., as discussed above. Also, U.S. Pat. No. 5,284,783 issued Feb. 8, 1994 to Ishikawa et al. discloses a heterojunction bipolar transistor that includes a graded InGaAlAs base layer and a graded InGaAlAs emitter-base transition layer.
What is needed is a high performance HBT that does not suffer from a low collector breakdown voltage or high output conductance, and offers other device performance capabilities, such as increased base transit times, for high power applications. It is therefore an object of the present invention to provide such a device.